1. Field of the Invention
The present invention relates to an antenna supervising apparatus, and more particularly, to an antenna supervising apparatus comprising a measuring unit for measuring a standing wave ratio (referred to as an SWR hereinafter) in order to supervise an impedance matching state between transmitters and an antenna. A voltage standing wave ratio is referred to as a VSWR hereinafter.
2. Description of the Prior Art
Recently, mobile radio communication systems such as an automobile radio communication system, an airplane radio communication system have been put into practical use accompanying developments of radio communication technique. FIG. 7 shows a conventional cellular radio zone type mobile radio communication system.
As shown in FIG. 7, base stations 2 are provided within respective small radio zones 1 corresponding divided service areas, and radio communications are performed between mobile stations 3 located within a small radio zone 1 and a base station 2 of the small radio zone 1 through communication channels 4 and control channels 5 used for transmitting calling signals. The respective base stations 2 are connected through an exchange control station 6 and an exchange control subsystem 7 to a fixed telephone network 9 connected to fixed telephones 8. In such cellular radio zone type mobile radio communication system, each cell, of the radio zone 1 is further divided according to increase in a number of subscribers. Then, according to the division of the cell, it is necessary to alter the frequencies to be used in the respective base stations 2, for example, using commands transmitted from the exchange control station 6.
As shown in FIG. 8, the base station 2 comprises plural transmitters Tx1, Tx2, . . . , Txi, . . . , Txn (i is a natural number which is equal to or larger than one and equal to or smaller than a natural number n) for respectively transmitting carrier waves having frequencies f1 to fn different from each other. The respective outputs of the transmitters Tx1, Tx2, . . . , Txi, . . . , Txn are outputted, through corresponding respective isolators I1, I2, . . . , Ii, . . . , In constituted by three terminal type circulators each having one terminated terminal, to corresponding respective band-pass filters F1, F2, . . . , Fi, Fn which are provided with the isolators I1, I2, . . . , Ii, . . . , In in a transmitter combiner 11, and are then band-pass-filtered. Thereafter, the outputs of the band-pass filters F1, F2, . . . , Fi, . . . , Fn are frequency-multiplexed, and a multiplexed wave comprised of respective carrier waves frequency-multiplexed is outputted through a directional coupler 12 for detecting a progressive wave transmitted to an antenna 13 and detecting a reflected wave transmitted from the antenna 13, and an antenna filter F0 of a band-pass filter for band-pass-filtering the multiplexed wave to the antenna 13, thereby projecting the multiplexed wave as a radio wave toward a free space.
In order to establish a certain reliability of the radio communication and to prevent occurrence of .radio interferences etc., a high frequency power projected from the antenna 13 toward the free space and an impedance matching state between a transmitter system and an antenna system are always supervised in the base station 2.
As further shown in FIG. 8, in an antenna supervising apparatus 100 provided in the conventional cellular radio zone type mobile radio communication system, a progressive wave transmitted to the antenna 13 and a reflected wave transmitted from the antenna 13 are detected by the directional coupler 12 which is inserted between the antenna filter F0 and the transmitter combiner 11, and the progressive wave and the reflected wave outputted from the directional coupler 12 are respectively detected by detectors 14 and 15 of diodes so as to obtain direct-current voltages which are respectively in proportion to the progressive wave and the reflected wave. The detector outputs of the progressive wave and the reflected wave are inputted to a calculation circuit 16, and then, in response thereto, a high frequency power projected from the antenna 13 to the free space and a VSWR of the antenna system comprised of the antenna 13 and a transmission line located from the directional coupler 12 to the antenna 13 are calculated by the calculation circuit 16. Thereafter, the calculated result is displayed on a display unit 17.
However, in the conventional antenna supervising apparatus 100, since the progressive wave and the reflected wave of the multiplexed wave comprised of respective carrier waves are detected by the directional coupler 12 comprising two coupling lines and are detected by the detectors 14 and 15, beat signals of frequencies (f1-f2), (f1-f3), etc. are superimposed in the outputs of the detectors 14 and 15. Due to the beat signals, the outputs of the detectors 14 and 15 change as a time passes as shown in FIG. 10, resulting in decrease in detection precision of respective levels of the above-mentioned progressive wave and reflected wave. Therefore, there is such a problem that detection precision of the high frequency power and the VSWR calculated by the calculation circuit 16 in response to the progressive wave and the reflected wave are lowered.
Further, when an interference wave having the other frequency is inputted from an external apparatus or system to the antenna 13, the interference wave is superimposed in the reflected wave, there is such a problem that the level of the reflected wave and the VSWR can not be correctly measured.